Science Assessment ~ Ideas ~ A pure substance has characteristic properties, such as density, a boiling point, and solubility, all of which are independent of the amount of the substance and can be used to identify it.

Key Idea: Whenever substances within a closed system interact with one another, the total mass of the system remains the same.

Sub-Ideas
Items & Student Performance
Misconceptions
NGSS

Students are expected to know that:

When substances mix, undergo chemical reactions, change state, or dissolve, or when objects are cut or broken into smaller pieces, the total mass of all the matter will always remain the same.
Regardless of the form that the products of these processes may take (for example, when a sugar cube dissolves in water or a chemical reaction produces a gas), the mass will always stay the same.
If it appears that the mass has changed, it is because some material has not been accounted for.
Because light is not matter, its presence or absence does not affect the mass of the matter.

Boundaries:

Students are not expected to know that mass is not conserved in energy-mass conversions such as nuclear reactions or other subatomic interactions.
Note: The words “weight,” “weigh,” and “mass” are used appropriately in the assessment items. The students are not expected to know the difference between “weight” and “mass.”

Percent of students answering correctly (click on the item ID number to view the item and additional data)
Item ID Number	Knowledge Being Assessed	Pre-Test	Post-Test
SB066001	Two liquids undergo a chemical reaction in an open jar and bubbles form. The mass of the liquids is less after the reaction because a gas was produced and that gas left the system.	62%	84%
SB065001	Mass is conserved when a plant dies in a sealed jar.	47%	75%
SC075004	When mold grows on a piece of bread in a sealed container, the bag and its contents weigh the same before and after the mold start growing.	18%	57%

Frequency of selecting a misconception
Misconception ID Number	Student Misconception	Pre-Test	Post-Test
SCM042	If something grows in a closed system, the total mass of the system increases. For example, when a tree grows in a closed system, the total mass of the system increases (Mitchell et al., 1984) or when mold grows in a closed system, the total mass of the system increases (DeBoer et al., 2009).	66%	36%
SCM029	Mass is not conserved during processes in which gases take part (Mas et al., 1987; Berkheimer et al., 1988; Hesse et al., 1992). If a gas is produced during a chemical reaction that takes place in a closed system, the total mass decreases (Ozmen et al., 2003).	15%	7%
M010	Mass always decreases during a chemical reaction	13%	7%
SCM010	Matter can disappear with repeated division, dissolving, evaporation, or chemical change (Smith et al., 2004).	10%	2%
SCM041	During biological decomposition in a closed system, the total mass of the system decreases (Mitchell et al., 1984).	8%	6%

Frequency of selecting a misconception was calculated by dividing the total number of times a misconception was chosen by the number of times it could have been chosen, averaged over the number of students answering the questions within this particular idea.

NGSS Statements
Code	Statement
PS1.B Grade 5	No matter what reaction or change in properties occurs, the total weight of the substances does not change. (Boundary: Mass and weight are not distinguished at this grade level.)